1. Field of the Invention
The present invention relates to a vertical deflection waveform generating apparatus for use in display units employing a CRT such as a television receiver, which enables control of, for example, a vertical width, vertical linearity, and a vertical position of a raster.
2. Description of the Prior Art
FIG. 13 is a circuit diagram showing a conventional vertical deflection waveform generating apparatus. In FIG. 13, reference numeral 1 means a vertical synchronizing signal which is input from an internal synchronous circuit or is externally input, 2 means a transistor which is turned ON or OFF by the vertical synchronizing signal 1, 3 is a current source, 4 is a resistor for current adjustment, 5 and 6 are capacitors for generating a sawtooth waveform, and 7 is a sawtooth wave generating circuit including the component parts 1 to 6.
Further, reference numeral 8 is a buffer amplifier, 9 is a vertical drive circuit, 10 is a vertical output circuit, 11 is a vertical deflection coil, 12 and 13 are resistors for correcting vertical linearity, 14 and 15 are respectively a variable resistor and a resistor for controlling a vertical position, and 16 is a resistor for detecting vertical deflection current. Reference numerals 17 to 20 are resistors and capacitors for generating S-shaped correction waveform, respectively.
When the transistor 2 is turned ON by the vertical synchronizing signal 1, the capacitors 5 and 6 are discharged through the transistor 2. When the transistor 2 is turned OFF, the capacitors 5 and 6 are charged by current I.sub.2 from the current source 3. At that time, current I.sub.1 is adjusted by the resistor 4 so as to adjust the current I.sub.2 serving as mirror current of the current I.sub.1. Subsequently, when the vertical synchronizing signal 1 is applied again, the transistor 2 is turned ON again so that the capacitors 5 and 6 are discharged.
The above operation is repeated to generate sawtooth voltage at a terminal of the capacitor 5. The sawtooth voltage is applied to the vertical drive circuit 9 through the buffer amplifier 8, and is compared with reference voltage V.sub.s for waveform shaping. Thereafter, the sawtooth voltage is further amplified by the vertical output circuit 10, and is applied to the vertical deflection coil 11. Thereby, sawtooth vertical deflection current 13 flows in the vertical deflection coil 11.
In order to perform S-shaped correction as vertical linearity correction to the vertical deflection current I.sub.3, charging current for the capacitor 6 is controlled by the resistor 13 in the first half of the S-shaped correction waveform, and charging current for the capacitor 5 is controlled by the resistor 12 in the latter half of the S-shaped correction waveform.
Further, in order to perform vertical position control of a raster, the variable resistor 14 is adjusted to control current I.sub.4 in the resistor 15. As a result, the vertical position of the raster can be adjusted. In addition to the conventional embodiment as set forth above, other related prior arts are disclosed in Japanese Patent Application Laid-Open Nos. 3-185489, 62-230167 and 63-99675.
The conventional vertical deflection waveform generating apparatus is provided as set forth above. Therefore, in order to manage modification or variation in the number of lines, in vertical timing and the like, it is necessary to provide plural sets of resistors 12, 13 for vertical linearity correction, and to select an appropriate set of resistors 12, 13 by a switch. Hence, there is a problem in that, for example, the number of assemblies is increased. Alternatively, it is considered possible to use output voltage of the S-shaped correction waveform so as to manage these modifications and the variations. However, since only a correction waveform having a predetermined pattern can be obtained, there are some problems in that, for example, it is not always possible to provide the optimal vertical linearity for all vertical frequencies, all timing, and all CRTs.